This invention relates generally to the preparation of substrates having a thermal-spray coating thereon, and more particularly to methods of testing of the integrity of a bond between the thermal-spray coating and the substrate.
At least some known metallic coatings are applied to substrates using a thermal-spray process in which a coating material, usually provided in a powder or wire form, is heated to an elevated temperature in a spray device. The coating material may be fully melted to form liquid droplets, may be partially melted to form semiplastic particles, or may remain formed as solid powder particles. The coating material is discharged from the spray device at a high velocity and towards a substrate surface. The sprayed material is deposited on the surface and, to the extent that it is liquid, solidifies. More specifically, droplets and particles impact the surface at a relatively high velocity, and are substantially flattened against the surface. The deposition continues until the solidified coating has reached a desired thickness.
The thermal-spray process is highly versatile and may be used with a wide variety of compositions and substrate articles. For example, the thermal-spray process may be used to deposit a coating on an article that has been partially worn away during prior service, wherein the coating has substantially the same composition as the substrate article. In another example, the thermal-spray process is used to deposit a wear-resistant coating across a surface, wherein the coating has a different composition than the substrate article and is more wear-resistant than the substrate article. In yet another example, the thermal-spray process may be used to deposit a wearing or abradable coating across a surface, wherein the coating has a different composition than the substrate article and is less wear resistant than the substrate article. Moreover, the thermal-spray process may be used to coat irregular and complexly shaped article substrates.
Generally, to be effective, the thermally sprayed coating must adhere at a bondline to the entire surface to which it is applied with a good mechanical bond. Accordingly, delaminations of the coating from the substrate may enable the coating to separate from the substrate. In some more-demanding applications, the coating must further be metallurgically bonded to the substrate.
At least one known method to determine the bonding strength of the bonding of the sprayed coating to the substrate requires destructive sectioning of the coated substrate article and metallurgical inspection of the bondline region. This method is normally used to establish process parameters that achieve a good bonded coating, and then the same process parameters are duplicated in the production coating operations. Because the thermal-spray process is so versatile, it may be difficult to perform destructive testing over the entire range of possible types of coatings and configurations of substrate articles. Moreover, even if a process is deemed through the destructive testing process, relatively minor variations in production parameters may lead to unacceptable bondline structures in the production articles. Another problem with the use of test articles is the test articles may behave differently than the production articles. Additionally, post-coating operations such as heat treating and machining may introduce bondline defects to initially defect-free bondlines.